Add Rust build metadata for out-of-tree kernel modules

This adds the needed build artifacts to compile out-of-tree kernel modules written in Rust. See https://github.com/Rust-for-Linux/rust-out-of-tree-module.

I can confirm that compiling the module works already with these changes, but I didn't yet test to actually load it (I'd need to reboot into that kernel). It works since quite some time in the https://aur.archlinux.org/packages/linux-rust kernel package, but there I compile with clang, not gcc.